1. Field of the Invention
The invention relates to electronic flash apparatus. More particularly, the invention relates to flash apparatus for producing flash illumination corresponding to pre-flash photographic conditions.
2. Description Relative to the Prior Art
The amount of flash illumination that should be emitted to produce a proper flash exposure is dependent on photographic conditions such as camera-to-subject distance, film sensitivity, ambient light intensity, and exposure aperture size. There is known in the prior art electronic flash apparatus that emit a variable amount of flash illumination from one exposure to the next corresponding to photographic conditions that are determined prior to exposure, rather than providing either a constant light output for all exposures or a variable amount of flash light based on light actually reflected from the photographic subject during exposure. For example, U.S. Pat. Nos. 2,901,671, 3,296,947, and 3,976,913 teach the charging of a flash-firing capacitor only to a given voltage relating to the energy required to produce flash illumination corresponding to pre-flash photographic conditions. So that a flash exposure is not delayed appreciably while waiting for the charging of the flash apparatus, the capacitor may be charged to some preliminary voltage before the actual photographic conditions are known. This preliminary voltage only grossly approximates the voltage that is required. Once the actual photographic conditions are established, the charge on the capacitor is adjusted--either increased or decreased.
To charge a capacitor only to a given voltage, whether or not charging is in separate phases, complicates the flash apparatus and adds further to its cost. Furthermore, flash-firing energy is wasted whenever the capacitor is caused to discharge for a purpose other than the production of flash illumination.
U.S. Pat. No. 3,682,056 teaches an alternative arrangement in which flash apparatus always charges a flash-firing capacitor to a full-charge condition, e.g. a predetermined, maximum voltage. For a proper exposure, flash illumination is quenched when the capacitor discharges to an adjustable voltage which is related to pre-flash photographic conditions.
Because a fully charged capacitor is not required unless a photographic subject is located at the maximum flash distance, an operator of the flash apparatus of U.S. Pat. No. 3,682,056 normally would not be able to take successive flash exposures as quickly as would otherwise be possible. Furthermore, leakage current from a capacitor varies in proportion to capacitor voltage. If fully charged flash apparatus is fired infrequently, excessive charge is wasted.